What is the Gunning Fog Index?
The Gunning Fog Index (FOG) is a readability formula developed by Robert Gunning in 1952. It estimates
the years of formal education needed to understand a text on first reading. The formula combines
average sentence length with the percentage of complex words (3+ syllables) to produce a grade level.
A score of 7–8 is considered ideal for general audiences. Scores above 12 are typically too complex
for most readers. Gunning Fog is widely used in business writing, publishing, and marketing.
How the Gunning Fog Formula Works
The formula is 0.4 × [(words ÷ sentences) + 100 × (complex words ÷ words)]. Complex words are defined as words with three or more syllables. So both sentence length and vocabulary difficulty (via complex words) affect the score. Gunning designed it to be easy to calculate by hand and to highlight "fog"—unnecessary complexity—in business writing. Passages of 100+ words give stable results.
When to Use Gunning Fog
Use Gunning Fog when you need a grade-level estimate for business writing, marketing copy, reports, or general-audience content. It is widely cited in publishing and corporate style guides. If your audience or client requires a specific formula (e.g. SMOG for healthcare or Flesch-Kincaid for education), use that one; otherwise Gunning Fog is a solid choice for general readability. To compare with other formulas at once, use our Combined Readability Checker.
Industry Guidelines and Targets
A Gunning Fog score of 7–8 is often recommended for general readers—roughly 7th to 8th grade. For broader accessibility, aim for 6–7. Scores above 10 suggest the text may be too complex for many consumers. Technical or academic content may legitimately sit at 12 or higher for specialized audiences. Check your organization's readability policy and use this calculator to stay on target.
How to Improve Your Gunning Fog Score
To lower the score (make the text easier), shorten sentences and reduce the share of complex words (3+ syllables). Replace multisyllabic terms with shorter synonyms where possible. Break long sentences into two or three shorter ones. One main idea per sentence helps. Because the formula weights both sentence length and complex words, edits in either area will move the score. Re-run your text after revisions to see the new grade.
How to Use This Calculator
Paste your text into the box at the top and click analyze. You will see your Gunning Fog score and a plain-English interpretation. No signup required. For best results use at least 100 words. To see how the same text scores on SMOG, Flesch-Kincaid, and seven other formulas, use the Combined Readability Checker.
Other Readability Tools
Get all nine formulas at once: Combined Readability Checker. Or explore individual calculators: Flesch-Kincaid Grade Level,
SMOG Index,
Dale-Chall Readability,
Automated Readability Index,
Coleman-Liau Index,
Linsear Write,
Lix,
Rix,
and the full Readability Calculators hub.
